6 Answers
6

I bet you use an encrypted home folder - which is readable not before you log in. What you can do is to copy your background image for instance to /usr/share/backgrounds (as root, i.e.: sudo cp ~/Pictures/backgroundimage.png /usr/share/backgrounds/) and set it as background image out of this un-encrypted folder.

Not sure what the problem in LightDM really is but i solved this by scaling my wallpaper to my screens resolution.
I moved them to /usr/share/backgrounds (location of the default wallpapers) as well. Not sure what did the trick but it works for me.